In many minimally invasive healthcare procedures, it is advantageous to track the medical devices such as guide wires and catheters. Optical shape sensing enables this by measuring and analyzing the light reflected from all positions of a multi core optical fiber built into the elongated medical device. When interferometer is used a full distribution of strain along the fiber is obtained, which can be converted into shape. A description of the method can be found in patent application US 2011/0109898 entitled “Optical position and/or shape sensing”. The backscattering of light in an optical fiber can be classified in two different classes: 1) an intrinsic phenomenon, viz. Rayleigh scattering, and 2) an extrinsic phenomenon, viz. Bragg gratings. Note that one then disregards a third scattering mechanism, viz. Brillouin scattering. Brillouin scattering cannot be employed in an interferometric measurement technique and leads to a poor spatial resolution. One implementation of optical shape sensing employs Rayleigh scattering. This has the advantage of using the optical fiber without additional steps in manufacturing after fiber draw.
Signal Strength of Rayleigh Scattering
The manufacturers of telecommunication fibers have increased the quality of their products over the years to such an extent that the transmission losses are small and mainly due to Rayleigh scattering. For shape sensing this has the disadvantage that the signal strength is rather low. In Annex I, its magnitude is calculated together with the ensuing signal to noise ratio. The small signal to noise ratio urges one to take additional precautions in the interferometric measurement system:
The reflection of the distal end of the fiber overwhelms all other signals, is too large and has to be reduced by at least 80 dB. In order to do so absorbing glass is fusion spliced to the distal end. This termination, however, is fragile and breaks of easily, so that integration of the fiber in medical devices has a low yield. Moreover, this kind of termination has the side effect that the last 5-10 mm of the fiber cannot be shape tracked.
A shape sensing fiber contains at least 4 cores. Each of the cores is connected to a separate interferometer. Therefore, the system contains an element where individual single core fibers are attached to one multi-core fiber: the fan out. At the fan out and at the termination of the sensing fiber cross talk arises: light that propagates down the fiber in a particular core is scattered at these points back into one of the other cores towards the detectors. This cross talk can be mitigated by configuring the four interferometers in a staggered alignment. See patent application US 2011/0310378 entitled “Interferometric measurement with crosstalk suppression”. The staggering of the interferometers entails the incorporation of substantial amounts of fiber in each of the interferometer arms. In total, the system will contain 500-600 m of additional fiber, making it more sensitive to temperature variations and mechanical vibrations.
Each of the interferometers has a built in optical circulator unit. With this S-port device the light from the source (port 1) is directed towards the sensing fiber (port 2), and the back scattered signal from the sensing fiber is directed towards the detectors (port 3). The light entering port 1 should not leak into port 3, otherwise is would overwhelm the true Rayleigh signal and saturate the detectors. The rejection ratio of most circulators is too low for sensing using Rayleigh scattering. Each circulator is tested and about 10% meets the proper specifications.
In minimal invasive healthcare applications, the interferometers will be built into a special vibration and temperature stabilized box. The medical device (catheter, guidewire) with its built-in shape-sensing fiber will be used in the sterile part of the Cath Lab. In between the two will be at least one optical patch cord. This means that there will be at least two optical multi-core connectors. Reflections at these connectors deteriorate the Rayleigh signals and are prone to give rise to cross talk effects. Current systems (e.g. Luna 3-rd generation) will not be able to function properly with more than one multi-core connector.
In between the interferometer box and the shape sensing fiber there will be additional lead wires. Those lead wires will also give rise to Rayleigh scattering, which will be of the same order of magnitude as the Rayleigh signal from the sensing fiber. The consequence is that those lead wires have to be interrogated as well and only after converting from spectrum as a function of optical frequency to signal as a function of delay, i.e. fiber position, the information from the lead wires can be discarded. Giving the finite sampling frequency and frequency sweep rate of the optical source, this gives an upper limit to the length of the lead wires.
Phase Retrieval of Rayleigh Scattering
Apart from the effects due to small signal strength, Rayleigh scattering has an enhanced sensitivity to mechanical vibrations as will be explained below.
A shape measurement comprises a scan over a wavelength range Δλ (e.g. 20 nm) around a central wavelength λb (e.g. 1540 nm). The spectrum is Fourier transformed resulting in a complex signal as a function of delay time, which is calibrated into position along the length of the fiber. The Fourier transformed signal is compared to a similar signal measured when the fiber was in a reference position, e.g. a straight line. In the comparison, the difference in phase (angle in the complex plane) of the two signals is taken at the corresponding position on the fiber. The slope of this phase difference as a function of position corresponds to the various strain components and can be translated into shape of the fiber. The integral of the strain at a given position is a measure of the total integrated length change of the fiber core. This length change means that the corresponding point of the reference measurement has shifted with respect to the shape measurement. Consequently, a shape tracking algorithm has to be employed that ensures the coherence between shape measurement and reference. Moreover, from measurement to measurement the starting point of reference measurement will be shifted with respect to the starting point of the current shape measurement owing to length changes in the patch cord. Length changes in the patch cord are caused by vibrations and temperature fluctuations. At the start of the phase tracking algorithm a cross correlation has to be performed in order to find the corresponding starting positions. There will be an upper limit in the allowed difference of the two corresponding positions on the fiber while still maintaining coherence of the phase signals. This upper limit will now be estimated and is a measure for the sensitivity of the system to mechanical vibrations and temperature fluctuations.
Rayleigh scattering originates from density fluctuations in the glass present at the moment of solidification and as a consequence has a random nature. In an interferometric set up the Rayleigh signal at a particular wavelength is a summation of all reflections along the length of the fiber. In the summation, the accumulated phase delay of each of the contributions is taken into account. This gives rise to a spiky nature of the interferometric spectrum. Its Fourier transform will also be spiky with a characteristic length scale δl that is a fraction of the wavelength in the glass. Fourier transformed spectrum however has a step size length Δz between consecutive points probed along the fiber of:
                              Δ          ⁢                                          ⁢          z                =                              λ            b            2                                2            ⁢            n            ⁢                                                  ⁢            Δλ                                              (        1        )            Here n is the group refractive index of the optical mode in the fiber. The characteristic coherence length δl (estimated to be on the same order of magnitude as the period probed i.e. λ/2n=500 nm) of the Rayleigh scattering is much smaller than the step size length Δz (approximately 40 μm or micrometer). FIG. 1 displays the phase of a measured and Fourier transformed Rayleigh signal. The phase is completely random even at length scales of a step size. Luckily, it reproduces so that phase tracking with respect to a reference signal is possible. FIG. 1 does, however, reveal that minute shifts in length will completely destroy the coherence of the phase difference between shape measurement and reference.Fiber Bragg Gratings
The solution to the above mentioned problems is to use an extrinsic scattering signal by writing Bragg gratings in the 4 cores of the sensing fiber. The scattering efficiency can be around 1% in magnitude, which is to be compared to the 10−8 of Rayleigh scattering (see Annex I). The signals of the interferometer will increase by the square root of this ratio, i.e. 103 or 60 dB. Termination of a shape sensing fiber needs only a small amount of suppression of the end reflection, so that e.g. an 8 degree angle polished cut will suffice. All issues concerning cross talk between fiber cores, finite rejection ratio of the circulator, reflections due to multi-core connectors are mitigated. Furthermore, the lead wires will have a negligible signal with respect to the shape sensing fiber. Increasing lead wire length can easily be compensated by adding equal amount of fiber length in the reference arm of the interferometer without deterioration of the integrity of the phase measurement.
U.S. Pat. No. 7,781,724 is an example of a shape/position sensing device using fiber Bragg gratings. The device comprises an optical fiber means. The optical fiber means comprises either at least two single core optical fibers or a multicore optical fiber having at least two fiber cores. In either case, the fiber cores are spaced apart such that mode coupling between the fiber cores is minimized. An array of fiber Bragg gratings (FBGs) are disposed within each fiber core and a frequency domain reflectometer is positioned in an operable relationship to the optical fiber means. In use, the device is affixed to an object. Strain on the optical fiber is measured and the strain measurements correlated to local bend measurements. Local bend measurements are integrated to determine position and/or shape of the object. An inherent disadvantage is that for typical FBG configurations, the detector of the reflectometer must have a relatively large dynamic range to encompass the information in the ‘wings’ of the spectral band.
The inventors of the present invention has appreciated that an improved shape and/or position sensing system is of benefit, and has in consequence devised the present invention.